The star, LP 876-10, wasn't discovered until 1980, and now it's found to be the third star of the Fomalhaut system, so it's Fomalhaut C. This discovery was long in coming, since LP 876-10 is 2.5 light years from Fomalhaut A and 3.2 light years from Fomalhaut B. For comparison, a third star, Proxima Centauri, presumed to be associated with the binary members of the Alpha Centauri star system is only 0.2 light years from its companion stars. Fomalhaut A is an A-class star, Fomalhaut B is a K-class star, and Fomalhaut C is an M4-classred dwarf star.

Star map of the region around Fomalhaut A and its companion stars, Fomalhaut B, and the newly discovered Fomalhaut C. (Base star map by Roberto Mura.)[6)]

This was a surprising result, since LP 876-10, now Fomalhaut C, is about 5.5 degrees away from Fomalhaut A. This separation, 0.77 parsec from Fomalhaut A and 0.987 parsec from Fomalhaut B, is still within the estimated 1.9 parsec gravitational radius of the Fomalhaut system. Not only that, but the motion of Fomalhaut C is within a kilometer/sec of its companions.[3]

Careful observation is a requisite for good science, and the discovery of Fomalhaut C is a good example. Says Mamajek

"I noticed this third star a couple of years ago when I was plotting the motions of stars in the vicinity of Fomalhaut for another study... However I needed to collect more data and gather a team of co-authors with different observations to test whether the star's properties are consistent with being a third member of the Fomalhaut system."[4]

One key to the discovery was knowledge of the actual distance of LP 876-10. As even a layman knows, a close separation of stars on the celestial sphere is no indication of the actual distance between the stars; and, in this case, the large angular separation of LP 876-10 from Fomalhaut A and Fomalhaut B masked its actual proximity. Jennifer Bartlett, while working on her Ph.D.thesis at the University of Virginia, had just measured the parallax of the star, so its distance was known.[4] Fomalhaut C may have evolved close to its companions, and it was then pulled out of the system by an interloper. However, this is not likely to have been the case, since the planetary system of Fomalhaut A would have been disrupted.